Power semiconductor devices should provide the minimum possible turn-on resistance, reverse leakage current and high switching speed at the rated breakdown voltage, to reduce the operational conduction loss and switching loss. The wide bandgap (Eg=3.26 eV), high threshold field of dielectric breakdown (2.2 MV/cm) and high thermal conductivity (4.9 W/cm-K) of silicon carbide (SiC) make it an ideal material for power switching devices. The thickness of voltage supporting layer (a low doping concentration drift layer) of power devices made of SiC is one-tenth of that made of silicon at the same rated blocking voltage, and the theoretical conduction resistance of SiC power devices can be hundreds times lower than Si power devices.
However, the wide bandgap of SiC also makes the turn-on voltage of body diode of SiC metal oxide semiconductor field effect transistor (MOSFET) reach to nearly 3V, which will result in a larger loss during switching and limit the switching speed. Furthermore, the basal plane dislocations happened during epitaxial growth of SiC drift layer will expand into stacking faults due to recombination of carriers during the forward conducting of body diode. SiC MOSFET's may degrade or even fail due to these stacking faults. Therefore, a SiC MOSFET sometimes co-packages an reverse-parallel connected SiC Schottky diode externally to increase the operating speed, reduce switching loss and avoid reliability issues brought by stacking faults.
In addition to externally connected with a Schottky diode, U.S. Pat. No. 6,979,863 discloses a SiC MOSFET integrated with a Schottky diode. In the SiC MOSFET of the above disclosure, the source metal and the Schottky metal are adjacent to each other which require additional layers in the manufacturing process to individually fabricate source contacts and Schottky contacts. Besides, to prevent the source metal erroneously contact to the drift layer and thus cause leakage current of the SiC MOSFET, greater tolerances in design rules need to be reserved to avoid yield loss. Thus, an effective gate width per unit area of the SiC MOSFET and a current density of the device may be undesirably affected, with costs further increased.